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EMB1499Q: Ringing on discharge startup

user3473222
Intellectual 300 points
Part Number: EMB1499Q
Other Parts Discussed in Thread: EMB1428Q,

Hello,

I am currently working with custom 12s active-cell balancing board. Balancing circuitry is made of two identical half-stacks (6s + 6s) based on EMB1428Q + EMB1499Q Chipset. Circuit was made according to TI's Active Chipset Reference Design. Th energy exchange is made between cell and the whole stack.

Both EMB1428Q in design successfully accepts SPI commands and charges-discharges selected cells. However during startup of discharge in lower half-stack a significant ringing occurs in both on both sides of converter: on stack side and on floating side (connected to the cell). The ringing can be seen on the attached graph below:

CH3 (purple) probe connected to PLUS_BOT (positive cell terminal)

CH4 (green) connected to MINUS_BOT (negative cell terminal)

MATH (red) is cell voltage between MINUS_BOT and PLUS_BOT (CH3-CH4)

As it can be seen, first of all negative cell terminal is connected (time at Cursor 1). Then after 156 us positive cell terminal is connected. And when converter turns on huge voltage swing can be seen. As I mentioned before, this behavior is observed when cell is discharged, and only in the bottom side of the stack. And it does not depend on which cell is selected.

Cell current is set to 1 A: Rsense is 0.01 Ohm and Vset is 400mV.

Which part of the circuit can be the source of this problem?


Schematics of the converter and switch matrix are attached below:

Bottom Switch Matrix.pdf

Bottom_Converter.pdfStack.pdf

  • 0
    TI__Genius 9401 points
    Hello,

    Thank you for your question. Can you please clarify what you mean by "only in the bottom side of the stack"? Does this mean that you only see the issue when discharging cells 1 to 6 but not on cells 7 to 12? If so, do you see the issue in every cell from 1 to 6?

    Best regards,
    Leslie
  • 0 in reply to Leslie Marquez
    Intellectual 300 points
    Hello,

    Thank you for reply. Yes I it happens when one of 1-6 cells are discharging. And i see this issue in every cell 1 to 6.

    While searching for the source of this issue i played a little with circuit. First of all I disconnected active clamp (soldered capacitors C70 and C77 out). At this point clamping is done by only 100V zenner (D32) circuit still works but noise seems the same. So it looks like active clamp is not the reason.
    Secondly I changed reference voltage Vset. To begin with it was set to 400mV (Icell = 1A). Then I increased reference to 1V (Icell =2.5A) and the ringing is few times smaller in amplitude. So it looks like low reference voltage somehow makes system unstable. Datasheet says that Vset should be in 0V - 2.2V range, but it also says that IC is tuned to work with Vset = 2V. So what is normal reference voltage range?
  • 0 in reply to user3473222
    TI__Genius 9401 points
    Hello,

    The Vset range in the EMB1499Q datasheet is actually from 1 to 2.2V (www.ti.com/.../emb1499q.pdf) as indicated in the "Operating Ratings" section on page 4, so 400mV is actually below this range. As you already mentioned above, the devices are factory trimmed to 2V, so you can get the best balance current accuracy at this voltage. Please refer to the "Application Hints" section on page 14 for details on component selection.

    As you initially mentioned that you are trying to use a balancing current of 1A, with Vset of 2V, your VSENSE_HS would be 50mV, which is within the range indicated on the Electrical Characteristics table on page 5. Then, you can calculate your RSENSE2 resistor (formulas can be found on "Application Hints" section.

    Best regards,
    Leslie